What is the actual send & track limit

Hello,

We have been attempting to use Send & Track but have found its limitations to be astonishingly low, and there is no clear indication of what the limitation actually is. We shared a document to 45 users (with unique links), which worked, but then a second document sent to another 45 users didn't work ("maximum" achieved).

Is the limitation 50 unique links? If I delete the existing shares, does that "free up" whatever the limitation is, or are those permanently used up until a certain time has passed?

This might not actually accomplish what we're trying to do but I need the limits for Send & Track to be actually & accurately spelled out because there's no indication anywhere in any documentation I can find of what they actually are.